TextBox input creates Exception : "Input string was...."

Hello :) 

I must say first that I'm new to C#. This code is not perfect but for now this is how I know to do it. I have 1 button 2 TextBoxes and 4 Lables, first 2 labels have some value and other 2 are result from addition in this case and it's working when I input values in both TextBoxes but when I try to input value in just one TextBox and click button I get Exception "Input string was not in a correct format.". So my question will be what I am doing wrong here ? How to make calculation for just one input value and not crash application? Thanks in advance.

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Code: 

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

namespace teST

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

        }

        public void Calculate(TextBox A, Label B, Label C)

        {

            int a = Convert.ToInt32(A.Text);

            int b = Convert.ToInt32(B.Text);

            int c = a + b;

            C.Text = Convert.ToString(c);

        }

        private void button1_Click(object sender, EventArgs e)

        {

            Calculate(tbX, lblX, lblResult1);

            Calculate(tbY, lblY, lblResult2);

         }

    }

}